Continuous production of lithium carbonate

Abstract

Lithium carbonate is derived from a calcined lithium-bearing silicate in a continuous cyclic process in which the silicate is decomposed in aqueous medium with sodium carbonate at a selected temperature and pressure, the resulting slurry is cooled and leached with carbon dioxide, and the resulting lithium bicarbonate-bearing soluton is heated to derive lithium carbonate and a mother liquor, which is recycled to the decomposition step. The amount of aqueous liquid used in the decomposition step is reduced to a minimum and the leaching accomplished without further dilution. This results in contamination of the recycled mother liquor with bicarbonate ions, which normally act to inhibit a high yield of lithium carbonate. The process conditions are controlled to reduce this inhibiting effect. Preferably, this control is effected by selecting a relatively low reaction temperature accompanied by neutralizing the mother liquor. Less desirably, the control is effected by selecting a relatively higher temperature and regulating it closely.

We claim: 
     
       1. A continuous cyclic process for producing lithium carbonate directly from calcined lithium-bearing silicates, comprising, continuously, reacting said silicate at a selected decomposition temperature and at a pressure higher than the water vapor pressure at the selected temperature with sodium carbonate in an amount of at least 1 mole of sodium carbonate per mole of lithium oxide present in the silicate and in the presence of an amount of an aqueous liquid from about 1.8 to about 2.5 times by weight of the amount of solids for a time effective to decompose the silicate to the point where substantially all the lithium in the silicate has been converted directly to the carbonate, thereby providing a mixture containing solid lithium carbonate and sodium aluminosilicate,   without increasing its liquid content cooling the mixture and leaching it with carbon dioxide at a pressure at at least atmospheric to provide a solid residue and a lithium bicarbonate-bearing solution,   separating the lithium bicarbonate-bearing solution from the solid residue,   heating and agitating said lithium bicarbonate-bearing solution to drive off carbon dioxide gas and to provide a precipitate of solid crystalline lithium carbonate and mother liquor containing traces of lithium, unreacted sodium and bicarbonate ions,   separating the mother liquor from the lithium carbonate and neutralizing it to reduce the bicarbonate ions substantially to a minimum,   returning the neutralized mother liquor to the decomposing step,   and recovering the lithium carbonate.   
     
     
       2. A process, as defined in claim 1, in which the selected temperature in the decomposition step is within the range from about 175° C. to about 250° C. and the pressure within the range from about 115 to about 620 psig. 
     
     
       3. A process, as defined in claim 1, in which the slurry from the decomposition step is cooled to a temperature within the range from about -10° C. to about 40° C. and the cooled slurry leached with carbon dioxide at a pressure from atmospheric to about 150 psig. 
     
     
       4. A process, as defined in claim 1, in which the pressure in the decomposition step is higher than the water vapor pressure at the selected temperature. 
     
     
       5. A process, as defined in claim 1, in which the carbon dioxide driven off in the insolubilizing step is recycled to the leaching step. 
     
     
       6. A process, as defined in claim 1, in which heat recovered from the cooling step is recycled to a previous step. 
     
     
       7. A process, as defined in claim 6, in which heat is recycled to the decomposition step. 
     
     
       8. A process, as defined in claim 6, in which heat recovered from the cooling step is applied to the insolubilizing step. 
     
     
       9. A process, as defined in claim 1, in which the insolubilizing step is carried out in a plurality of stages of increasing temperature whereby the amount of water evaporated is reduced substantially to a minimum and the water vapor in the carbon dioxide removed is reduced. 
     
     
       10. A process, as defined in claim 9, in which, in each stage, there is a temperature increase of at least about 20° C. 
     
     
       11. A process, as defined in claim 1, in which hot slurry from the decomposition, is submitted to a selective decantation to eliminate a major part of the residual sodium alumino-silicate without significant loss of lithium carbonate. 
     
     
       12. A process, as defined in claim 11, in which the hot slurry is set at a temperature between about 75° C. and about 95° C. and from about 65% to about 90% of the residual sodium alumino-silicate is eliminated. 
     
     
       13. A continuous cyclic process for producing lithium carbonate directly from calcined lithium silicate, comprising, continuously, reacting said silicate at a selected decomposition temperature within the range from about 175° C. to about 250° C. at a pressure within the range from about 115 to about 620 psig in an amount of at least 1 mole of sodium carbonate per mole of lithium oxide present in the silicate and in the presence of an aqueous liquid in an amount of from about 1.8 to about 2.5 times by weight of the solids for a time effective to decompose the silicate to the point where substantially all the lithium in the silicate has been converted directly to the carbonate, thereby providing a mixture containing solid lithium carbonate and sodium aluminosilicate,   without increasing its liquid content, cooling the mixture to a temperature within the range from about -10° C. to about 40° C. and leaching the cooled slurry with carbon dioxide at a pressure from atmospheric to about 150 psig to form a lithium bicarbonate-bearing solution and a solid residue and separating the solution from the solid residue,   heating and agitating said lithium bicarbonate-bearing solution in at least two stages of increasing temperature to drive off carbon dioxide gas to provide a precipitate of solid crystalline lithium carbonate and mother liquor containing traces of lithium, unreacted sodium and bicarbonate ions,   separating the mother liquor from the lithium carbonate and neutralizing it to a pH of at least 11.5 to reduce the bicarbonate ions substantially to a minimum,   and recovering the carbon dioxide driven off from the insolubilizing step and recycling it to the leaching step.   
     
     
       14. A process, as defined in claim 13, wherein the hot slurry from the decomposition is cooled to a temperature of from about 75° C. to about 95° C. and subjected to selective decantation to remove from about 65% to about 95% of the residual sodium aluminosilicate. 
     
     
       15. A continuous cyclic process for producing lithium carbonate directly from calcined lithium-bearing silicates, comprising, continuously, reacting said silicate at a selected decomposition temperature with an excess of sodium carbonate in the presence of a substantial minimum of water required to form an aqueous slurry for a time effective to decompose the silicate to the point where a major part of the lithium in the silicate has been converted directly to the carbonate, thereby providing a mixture containing solid lithium carbonate and sodium aluminosilicate,   without increasing its liquid content cooling the mixture and leaching it with carbon dioxide at a pressure at at least atmospheric to provide a solid residue and a lithium bicarbonate-lithium solution,   separating the lithium bicarbonate-bearing solution from the solid residue,   heating and agitating said lithium bicarbonate-bearing solution to drive off carbon dioxide gas and to provide a precipitate of solid crystalline lithium carbonate and mother liquor containing traces of lithium, unreacted sodium and bicarbonate ions,   separating the mother liquor from the lithium carbonate and neutralizing it to reduce the bicarbonate ions substantially to a minimum,   returning the neutralized mother liquor to the decomposing step,   and recovering the lithium carbonate.